Data storage devices of the type known as Winchester disc drives are well known in the art. Such disc drives magnetically record digital data on one or more rigid recording discs that are rotated at a constant, high speed. An array of data transducing heads access data tracks defined on the various disc surfaces to write data to and read data from the discs.
An actuator is used to controllably position the heads adjacent the discs through the use of a voice coil motor (VCM). A servo circuit uses prerecorded servo positioning data to provide closed loop head positioning. A digital data channel controls the writing of input data to data sectors defined on the discs and the subsequent recovery of the input data from the discs.
With the continued trend of achieving ever higher areal data recording densities on the discs, it is important for disc drive designers to be able to accurately characterize error rate performance of various data channel configurations in order to select a final configuration for a disc drive product that achieves the requisite performance characteristics. Disc drive designers often use a measure referred to as raw bit error rate (RBER) to characterize channel performance.
RBER is defined as a ratio of the number of erroneous bits out of a total number of bits read. It has generally been accepted historically that a smaller RBER generally correlates to better drive performance, including a lower probability of outputting data to a host device containing undetected errors (probability of passing undetected erroneous data, or PUED).
A limitation with this focus on RBER is that, with recent advancements in data processing technology, it has been discovered that a lower RBER does not necessarily correlate to improved performance. That is, other factors including error correction coding (ECC) schemes, run length limited (RLL) encoding schemes, and the interaction that can take place between these approaches can make RBER less than a valid predictor of overall drive performance. In fact, in some encoding schemes where redundant levels of encoding are also applied, RBER becomes a meaningless measure of system performance or at least inaccurate indicator of quality.
As will be recognized, ECC schemes allow the detection and correction of selected numbers of erroneous symbols in a data stream through the calculation of appropriate code words which are appended to the user data and recorded to the discs. Different powers of ECC can be employed based on different symbol lengths (i.e., number of consecutive bits in each symbol) and numbers of interleaves (i.e., groupings of the symbols in an order different from that in which the bits appear on the media). Exemplary symbol lengths can include 8 bits, 10 bits, 12 bits, 14 bits, etc.
RLL schemes and (and other constraint-based encoding schemes) are generally used to ensure that logical 1's (flux transitions) in the bit stream fall within certain bounds of occurrence. A typical RLL scheme specifies both the minimum and maximum number of logical 0's that can occur between consecutive logical 1's. RLL schemes involve a transformation of m bits into n bits. Exemplary RLL schemes include 8/9, 16/17, 32/34, 99/100, etc. The higher the values of m and n, the lower the overhead; for example, using 8/9 encoding means that 9 bits are written to the disc surface to represent 8 user data bits, so that 1/9 (11%) of the data bits are overhead (nondata). Using 99/100 encoding, by contrast, means that every 99 user data bits are represented on the media by 100 bits, so that only 1/100 (1%) constitutes overhead. While higher RLL ratios advantageously reduce overhead, higher RLL ratios also tend to provide reduced signal to noise levels, so that higher RBER rates can be observed with higher RLL schemes.
Added to the difficulty in predicting performance of a disc drive design includes the fact that different head and media combinations and channel circuitry from different suppliers can provide different drive performance for a given encoding scheme. Disc drive development cycles are now often measured in months, if not weeks, and it is often impractical to wait to obtain physical samples of new channel circuitry from a supplier before evaluation of such circuitry can take place to decide on a final configuration for a given product.
Data integrity is a major factor in drive development. An accurate measurement for unrecoverable error events allows the disc drive designer to optimize encoding schemes and EGG algorithms for the disc drive recording medium. There is therefore a significant need for improvements in the art to enable a disc drive designer to readily and accurately characterize different alternative configurations of a digital data channel, including the use of different RLL and ECC encoding schemes, symbol lengths and interleaves.